Located in various offices of many businesses are data terminals connected to a common data processing center. The offices may be in the same building, several adjoining buildings or scattered throughout a metropolitan area. In the past this arrangement has had a number of disadvantages when data terminals are moved or added.
Turning first to the situation where the data terminals and the data processing center are located near one another, i.e., in the same building or adjoining buildings; each time a data terminal is moved or added, either new wires for connecting the data terminal to the data processing center must be run or existing wires moved. Both running new wires and moving wires are time consuming (and, therefore, expensive) tasks. Moreover, running new wires and moving wires are often disruptive, because offices other than the ones directly associated with the new or moved data terminal are often disturbed as wires are strung or moved in ceiling, wall and other passageways.
Not only is the running of new wires and the moving of old wires disruptive of other offices, it may also delay the use of vacant offices where the data terminal(s) are to be located. That is, the time required to wire or rewire vacant space to accommodate data terminals must be included in the timing of a move into the space. As with other factors affecting a move, the availability of personnel, material etc., can create time delays even if the wiring or rewiring is scheduled to coincide with other changes. Such delays can contribute to delays in the occupancy of the vacant space. Consequently, there is demand for a data transmission system that allows the data terminals of a data processing system to be readily added or moved without requiring the running of new, or the moving of existing, wires used to connect the data terminals to the data processing center.
Turning next to the situation where the data terminals are separated from the data processing center by a substantial distance; such a situation may exist where a number of branch offices all associated with a central or "home" office. Such a situation exists in many commercial environments, e.g., banking real estate, etc. In such instances, the branch offices and the central or home office include data terminals connected to a data processing center located at one of the offices, which is usually the central or home office, but may be one of the branch offices or even an entirely separate office. In any event, as branch offices are opened and closed for various reasons data terminals must be moved. Further, the location of data terminals in the various offices is sometimes changed. This situation is somewhat different than the foregoing situation where the data terminals are located relatively near to the data processing center. Specifically, in the situation where the data terminals are separated from the data processing center, the terminals are usually connected to the center via telephone wires. As a result, when data terminals are to be added or moved, the owner or lessee of the data terminal is not required to add or move wires. On the other hand a related problem does exist in this situation. Specifically, when data terminals requiring dedicated (to the terminal) telephone wires are added or moved, the telephone company (TEL CO) must change the telephone service. This may simply require connecting up existing unused wires, or adding new wires. In either situation, the owner or lessee of the data terminal must pay for the additional or changed service. The use of dedicated telephone wires has a further disadvantage. Specifically, telephone wires dedicated to data terminals cannot be used for audio communications. As a result, separate data and audio communication wires are required for each office location requiring a data terminal. Since each pair of telephone wires is leased, the users costs are increased. (A somewhat similar situation exists where the data terminals are connected to telephone wires via a coupler, as required, rather than using dedicated telephone wires because of the additional telephone capacity required when the data terminal is using the telephone wires.)
Many businesses that have a data processing system wherein a plurality of data terminals are located in the vicinity of the data processing center have a private automatic branch exchange (PABX) telephone system. As with a TELCO central office (CO), the PABX routes telephone conversations from, to and between the business's telephone stations. Usually the telephone stations are widely scattered throughout the various offices of the business so as to be readily available to the business's employees. Consequently, usually a telephone station is located in the vicinity of each data terminal.
In view of the foregoing discussion, it will be apparent that the present method of transmitting data between data terminals and a data processing center has a number of disadvantages. The nature of the disadvantages varies in accordance with the distance separating the data terminals from the data processing center. It will also be apparent that telephone stations are usually located in the vicinity of data terminals. However, in the past the use of such telephone stations to transmit digital data has been limited due to the high costs involved in such utilization. The invention is directed to economically utilizing this available transmission medium.
Thus, it is an object of this invention to provide a new and improved digital data transmission system.
It is another object of this invention to provide a digital data transmission system suitable for transmitting digital data over telephone wires without interfering with the simultaneous use of the same wires to carry base band telephone signals.
It is a further object of this invention to provide a transmission system for transmitting digital data from data terminals to a data processing center on telephone wires without impairing the use of the telephone wires to simultaneously carry baseband telephone signals.